Phosphors are photoluminescent materials. A performance of a phosphor can be measured at the time of the phosphor's production and then reassessed over a period of time. The change in performance of the phosphor over time is a measure of the reliability of the phosphor's performance. The performance of a phosphor can be affected at the time of production or during use, for example by contamination of the phosphor with undesired elements, for example due to continued exposure to heat, oxygen, and moisture. Although some phosphors are considered to have a better reliability and thermal stability than others, the performance of these phosphors over time, i.e., their reliability, can be detrimentally affected by such continued exposure to heat, oxygen, and moisture during use.
Photoluminescent materials are integral components of white light emitting diodes (LEDs) which are typically used as backlight sources of various display sources including, for example, mobile phones and liquid crystal display devices. More recently, white-light-emitting LEDs using photoluminescent materials have been extensively used in lighting and proposed as substitutes for conventional white light sources, such as incandescent, fluorescent and halogen lamps.
Accordingly, one of skill will appreciate having highly reliable, coated, nitride-based red phosphors that can be used, for example, in LEDs. A desirable phosphor could have (i) a low oxygen contamination at the time of manufacture to provide (ii) a high initial performance, and (iii) a uniform inorganic compound layer, such as a silicon-dioxide coating to maintain a high performance over an extended period of time in a harsh environment.
Furthermore, phosphors can be coated with metal oxides to improve their reliability by serving as a barrier layer. The coatings, however, also vary in performance and reliability in themselves. For example, coatings can vary in porosity, can be more or less amorphous or hygroscopic than others, can have varying uniformities or morphologies, can be more physically susceptible to damage during use, for example, more apt to crack under use, or the like. Moreover, phosphors can not only serve as a barrier to the entry of contaminants, but they can absorb water or another contaminant, or perhaps result in a leaching of important phosphor components, during manufacturing of the coated phosphor, for example, which can affect the performance and/or reliability of the phosphor. In addition, since moisture is a contaminant that can cause phosphor degradation and color change, one of skill would appreciate a coating combination that results in a reduction in the amount of moisture reaching the phosphor and a reduction in leaching of chemicals from the phosphor to provide coated phosphors having an improved performance and/or reliability.
Accordingly, one of skill will appreciate phosphors having combination coatings with multifunctional characteristics that increase the performance and/or reliability of the phosphor. Such phosphors can be used, for example, to increase the reliability of products using LEDs. A desirable phosphor could have a coating that includes (i) a primer material having a primer thickness that inhibits or prevents a leaching of chemical from the phosphor; and (ii) a sealant material having a sealant thickness to inhibit or prevent the contamination of the phosphor from the operating environment, the combination primer and sealant materials providing a multifunctional coating that maintains a high performance over an extended period of time in a harsh environment.